1. Field of the Invention
This invention relates to the use of the relative magnetization orientation of sense and reference layers of magnetic tunnel junction (MTJ) devices (cells) to provide a logic storage function for such devices in non-volatile memory cell arrays. In particular it relates to the design and fabrication of cells that include a keeper structure providing a flux closure path to direct demagnetization fields away from the sense layer and thereby improve the thermal stability of the cell.
2. Description of the Related Art
The magnetic tunnel junction (MTJ) basically comprises two layers of ferromagnetic material, a sense layer and a reference layer, separated by a tunnel barrier layer, which is a thin layer of insulating material. The tunnel barrier layer must be sufficiently thin so that there is a probability for charge carriers (typically electrons) to cross the layer by means of quantum mechanical tunneling. The tunneling probability is spin dependent, depending on the orientation of the electron spin relative to the magnetization direction of the ferromagnetic layers. Thus, if these magnetization directions are varied, the tunneling current will also vary as a function of the relative directions for a given applied voltage. As a result of the behavior of an MTJ, sensing the change of tunneling current for a fixed potential can enable a determination of the relative magnetization directions of the two ferromagnetic layers that comprise it. Equivalently, the resistance of the MTJ can be measured, since different relative magnetization directions will produce different resistances.
The use of an MTJ as an information storage device requires that the magnetization of at least one of its ferromagnetic layers, the sense layer, can be varied relative to the other, the reference layer, and also that changes in the relative directions can be sensed by means of variations in the tunneling current or, equivalently, the junction resistance. In its simplest form as a two state memory storage device, the MTJ need only be capable of having its magnetizations put into parallel or antiparallel configurations (writing) and that these two configurations can be sensed by tunneling current variations or resistance variations (reading). In practice, the ferromagnetic sense layer can be modeled as having a magnetization which is free to rotate but which energetically prefers to align in either direction along its easy axis (the direction of magnetic crystalline anisotropy). The magnetization of the reference layer may be thought of as being permanently aligned in its easy axis direction. When the sense layer is anti-aligned with the reference layer, the junction will have its maximum resistance, when they are aligned, the minimum resistance is present. In typical MRAM circuitry, the MTJ devices are located at the intersection of current carrying lines called word lines and bit lines (or word lines and sense lines). When both lines are activated, the device is written upon, ie, its magnetization direction is changed. When only one line is activated, the resistance of the device can be sensed, so the device is effectively read.
As such cells become increasingly small in cross-sectional area and volume, the stability of the magnetization direction of the sense layer is easily affected by random external fields, its self-demagnetizing fields and by thermal effects. One approach to increasing cell stability is to provide the cell with a “keeper” structure, whose role is to provide a flux closure path that directs demagnetization fields away from the cell, thereby enlarging its effective volume. Anthony (U.S. Patent Application Publication No.: US2002/0055190 A1) provides a keeper structure which is a soft magnetic material surrounding a current carrying conductor beneath the sense layer. The structure thereby provides a mechanism for preventing the formation of demagnetization fields in the edge region of the sense layer above it. Anthony (U.S. Pat. No. 6,358,757 B2) provides a method for forming an array of MRAM devices at the intersections of orthogonally crossing upper and lower conductors in which the lower conductors are surrounded by soft magnetic keeper layers. Tuttle (U.S. Pat. No. 6,413,788 B1), within a variety of embodiments, teaches methods for forming keeper structures around both upper and lower conductors in damascene type trench configurations. In one pertinent embodiment, a tunneling magnetoresistance (TMR) structure (called, by the inventor, the “bit region”) is formed between two orthogonally positioned shielded damascene conductor elements. Tuttle (U.S. Pat. No. 6,417,561 B1) also provides a magnetic memory device formed by the methods of the previous cited patent.
A problem arises with the fabrication of such keeper structures in that it is difficult to maintain alignment between the keeper and the sense layer during the fabrication of the cell. Tuttle (in two patents cited above) discloses a keeper structure which is self-aligned with an upper conductor, but the upper conductor is not self-aligned with the cell junction (MTJ). In accord with the method of Tuttle, two separate photo-processes are required to form the device. The purpose of the present invention is to provide a novel design and fabrication methodology which includes the self-aligned formation of the top conductor and its keeper with the MTJ cell. This process provides an effective flux closure for shielding the sense layer of the cell in a manner that will permit the scaling down of cell formation to very high density MRAM formations.